Sound detection



1,470,733 H. C. HAYES SOUND DETECTION Filed June 25, 1919 3 Sheets-Sheet 1 IN VEN TOR. 7/1 I0 i i i i 2% Oct. 16', 1923.

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H. c. HAYES SOUND DETECTION Filed June 25 1919 3 Sheets-Sheet 3 INVENTOR. j/iC/Alyda H. C. HAYES SOUND DETECTION 3 Sheets-Sheet 5 Filed June 25 1919' FIG. 6.

FIG. 5.

FIG. H.

OPyQOOEIDOOOO IZJ Patented Oct. 16, 1923.

UNITED STATES 1,470,733 PATENT OFFICE.

HARVEY C. HAYES, OF NEW LONDON, CONNECTICUT, ASSIGNOR TO SUBMARINE SIGNAL COMPANY, OF PORTLAND, MAINE, A CORPORATION OF MAINE.

SOUND DETECTION.

Application filed June 25, 1919. Serial No. 306,688};

To all whom it may concern:

Be it known that I, HARVEY C. HAYES, a citizen of the United States, residing at New London, in the county of New London and State of Connecticut, have invented new and useful Improvements in Sound Detection, of which the following is a specification.

The present invention relates to sound detection. One feature of the invention relates to an arrangement of a. plurality of microphones spaced in a row and provided with compensated connections between the microphones and the ear for determining the direction of the sound by bringing the im pulses from the several microphones into phase. Other features of the invention relate to the structure of the microphone housing and to a towing device for trailing the microphones behind a ship. Still other features of the invention relate to certain arrangements and combinationsof parts hereinafter more particularly pointed out, the advantages of which will be ap arent to one skilled in this art from. the ollowing description.

Referring to the drawings, Fig. 1 is a diagrammatic view showing a multi-unit microphone line with compensated connections. *ig. is a side elevation of a sound detecting device usually designated as an eel. Fig. 3 is a side elevation in section of the head of the eel. Fig. 4 is a vertical section through the eel at one of the microphone units. Fig. 5 is an elevation partly in section of the tail of the eel. Figs. 6 and 7 are side and end elevations respectively of one of the microphone housings. Fig. 8 is a,

vertical section through the microphone.

housing. Fig. 9 is an end elevation of the microphone. Fig. 10 is a side elevation artially in section of the microphone. ig. 11 is a diagrammatic view illustrating a multi-unit microphone line as applied to the side of a ship.

The direction of a sound source may be determined by the binaural sensation of hearing. If the sound waves reach the right ear first, the sound is heard on the right side. It the sound waves reach the two ears at the same time. the sound is heard in a dircction at right angles to the line joining the two ears, that is to say either straight ahead or straight behind. The direction of sound travelling through the water may be similarly determined by binaural listening. If two submarine receivers are spaced about 5 ft. apart in the water and are connected respectively to the right and left ear of the observer and the sound waves strike the right receiver first, the sound will be heard as coming from the right by the observer. If the two submarine receivers are turned about each other so that the line joining them is broadside to the sound, the sound will appear to be centered in the head of the observer or appear to be heard directly ahead or behind. The manipulation of the submarine receivers to bring the sound waves to the ears at the same time is known as binaurally centering the sound and is used to determine the direction from which the sound is coming. The direction of the sound may be determined binaurally by having the two receivers in a fixed position in the water and compensating the paths of the received waves from the receivers to the ears of the observer. For example, if two submarine receivers are spaced apart under water and are connected to the right and left ears of the observer respectively and the sound approaches from such a direction as to strike the right hand receiver first and consequently appears to be heard at the right, the sound may be brought to a binaural center by introducing in the connection between the right receiver and the right ear, enough time lag so that the impulses from the two receivers will reach the two ears of the observer at the same time and thus give the impression that the sound is heard directly ahead. In case the two rece vers are connected to the ears by air columns, one air column may be lengthened to introduce the necessary time lag for binaural centering. By means of a suitable calibrated scale connected with the means for introducing time lag into the connections, the angular direction of the sound waves with respect to the base line between the receivers may be read directly.

If a number of receivers are employed, the direction of the sound may be determined by focusing. by means of which the sound is heard at a maximum when the sound waves are received by the several receivers and brought together in phase at the ear. Suppose a number of receivers are spaced in a straight row and that the receivers are all connected to the car by wave conducting leads or paths requiring the same time for the waves to pass over them. Then if a sound wave strikes the row of receivers broadside the sound waves as brought to the ear from the several receivers will be in phase and the sound heard will be at a. maximum. If the sound strikes the row of receivers at an angle the sound waves from the difierent receivers will reach the ear out of phase and the intensity of the sound heard will be cut down by interference. The direction of the sound may be determined by bodily turning the row of receivers until the sound is a maximum. The same effect may be had by having a row of receivers fixed in direction and introducing the proper amounts of time lag into the several leads from the receivers to the ear so that a wave front which strikes the row of receivers at an angle will have the waves conducted along the leads from the several receivers brought into phase at the ear. 7 I

The direction of sound may be determined by combining focusing with bina-ural centering, by having a row of receivers with some of the receivers connected to one ear and others connected to the other car.

In determining the direction of submarine sounds it is desirable to employ in many cases microphones as the sound receiving devices. If microphones are employed they may be installed in a device towedfrom the ship and thus be considerably isolated from the ships own noises, the connections to the compensator being made through a cable.

If the listening devices are mounted on the ships hull or within any of the various inside tanks of the ship, microphones have an advantage in thatthey can be readily installed and in that the compensator by which the direction of the sound is determined may be placed anywhere on the ship and at a distance from [the microphones.

Referring to the drawings, Fig. 1 shows diagrammatically a row of microphones and compensator connections for determining the direction of a sound wave.

The microphones indicated by numerals 1 to 12 inclusive are disposed in a row with predetermined spacing between the microphone units. The row of microphones is fixed in direction, as for example being arranged longitudinally in a. device towed behind the ship or arranged in a row along the ships hull. The microphones are con-- nected to a compensator indicated generally by reference numeral 13. The compensator has a number of receivers indicated by reference characters 1. 2, 3, etc.. to 12. These receivers resemble the Ordinary telephone receiver and convert the undulations of the electric microphone current into sound waves. One of the receivers is shown diagrammatically in cross section as comprismg a magnet 30 which actuates the diaphragm 31 which causes sound waves in the air column below it. The microphones have a common connect-ion 14 which includes the battery 15. The microphone is connected to its receiver 1 by means of the lead wire 1 and the transformer 1". The other microphones are similarly connected with their respective numbered receivers: The receivers l. :2. 3 etc. set up sound vibration in the air column in the telescopic tubes 1. 2". 3 etc. The ends of the telescopic tubes are connected to a lever 20 pivoted at 21 so that the lengths of the air columns in the tubes 1, 2, 3 etc. may bevaried. The several telescopic tubes are connected by leads 1", 9 3 etc. to the collecting tubes 22 and 23 and thence to stethescope ear pieces 24 and 25 for the left and right ears respectively. The several leads 1 2, 3 are of equal length, as are the tubes 22and 23.

In constructing a microphone line as here indicated the microphones are first matched, that is. the microphones are tested and microphones having the'same time constants are chosen so that if sound waves which are in phase strike the several microphones simultaneously the electric currents set up in the respective microphonecircuits will also be in phase. If the time constants of the microphone are not the same then the currents set up by one microphone will la; behind or be in advance of the currents in the other microphones and the waves cannot same electrical time constants so that vary ing lag will not be introduced into the different electric circuits.

Assume that the sound comes from a direction at right angles to the row of microphones 1-12 inclusiv waves as collected by the several microphones will be in phase and if the lever 20 is turned 'to a horizontal position so that the telescopic tubes 1, 2, etc. are of the same length, the sound Waves will be brought into focus at the. ears of the observer, so that the sound will be heard as a maximum. Also the time of arrival of the waves at the right and left cars will be the same so that sound will appear to be binaurally centered. The observer noting the position of the lever 20 will therefore know that the sound is coming in a direction at right angles to the base line of the row of microphones. I

Suppose, however, that the sound is coming at an angle. other than perpendicular, the sound wave having a wave front as indicated by the dotted line S in Fig. 1. The

Then the sound sound wave will strike the microphones 1, 2, 3, etc.. successively so that if-the telescopic tubes 1, 2, etc. are of the same length. the sound waves will interfere and a maximum sound will not be heard. Wore-over since the wave front strikes the left hand side of the line first. the sound will not be binaurally centered but will appear to be in the left ear. The sound at the ears may however be brought to a maximum and also binaurally centered by turning the lever :20 and thereby lengthening the telescopic tubes at the left hand end of the lever and shortening them at the right hand end. The ratio of the velocities of sound in air and water is approximately 23 to 100. The sound wave in water will be delayed in arriving at the microphone 2 after striking microphone 1 by the time necessary to travel the distance 03. If the length of the air column in the telescopic tube 1 is increased over the length of the air column in the telescopic tube 2 by an amount 23/100 (Z, then th sound waves striking the microphones l and 2 will be brought into phase by the air columns in the telescopic tubes. The same is true of the sound waves striking the other microphones. if the several telescopic tubes are relatively lengthened or shortened in respect to each other as shown in the diagram, and the waves of the wave front F as received at the several microphones will all be brought into phase at the ears and the sound will be heard as a maximum. Also since the sound waves are in phase at the two cars the sound will be binaurally centered. By calculating the lengths of the air column and by a suitable calibrated scale 40 adjacent to the lever 20. the observer may read directly the angle of incidence of the sound with respe t to the base line of the row of microphones.

The action of the plurality of microphones is not only to bring the sound waves from the source set upon by the compensator into phase and thereby cause this sound to be heard with a maximum intensity. but is also to diminish the sounds heard coming from sources at other angles. Suppose for example the compensator is set to listen to sound coming from a particular source and having the wave front S as shown in Fig. 1, and suppose that sound from a source located at another angular bearing is also present. The sound impulses from the second sound source as received by the several microphones and brought together at the collecting tubes will be out of phase and therefore will tend to neutralize each other by interference and such sound will be heard but faintly by the observer, and will cause slight. if any, interference in making of a binaural setting on the first sound. The ability to eliminate sounds from extraneous sources which is possessed by a line having several microphones from which the sound impulses are combined, gives it a distinct advantage over a listening device in which but a single microphone is connected to each car. because in the latter case sounds from all directions are heard \viththe same intensity. This suppression of other sounds is of particular advantage in submarine listening for determining the direction of a particular ship when there are other ships in the sound field. Moreover. a listening device in which a plurality of microphones are connected to each car has a range in any desired direction greater than a listening device in which but a single microphone is connected to each ear for the reason that the bringing of the impulses from the several microphones into phase increases the amplitudes.

For the sake of explanation, an electricair compensator having telescopic tubes is illustrated in Fig. 1. because the principle of compensation can best be explained by it.

The necessity of properly matching the microphones so that they will have substantially the same time constant should be cmphasized in constructing a multi-unit focusing microphone line. It is apparently much more important that the microphones be matched to have the same time constants than it is that the microphones should all be of the same loudness.

In addition to having the microphones matched. the microphone housings should be designed so as not to introduce unequal time constants in transmitting the sound from the water through the flexible diaphragms to the microphones. and so the several receiving units, each including the microphone and its sound receiving diaphragnn will have the same time constants. If all the units are properly matched the currents set up by the microphones will all be in phase in case the wave front strikes all the microphones simultaneously, or will lag be hind each other by equal amounts in case the wave front comes at such an angle as to strike the several n'iicrophones successively. The microphones and housings illustrated in Figs. 6 to 10 have been found by repeated experiments to be capable of successful use in making a microphone line in which the several units have the same time constants. In Figs. 2 to 10 the microphones are shown as arranged in a line carried by a device designed to be towed from a ship.

The device as a whole is indicated at 50 in Fig. 2 and is ordinarily termed an cel. The eel consists of a hollow flexible tube fit of rubber. In practice, eels containing twelve microphones have been made aboutfourteen feet long with twelve inch spacing between thev microphones and of soft rubber tubing about four inches in outside diameter. The eel is provided with a stream line nose or head 2 and is towed through the water by a cable 53 which contains the electrical leads to the nncrophones-:.

in case twelve microphones are used. the

cable should contain thirteen leads. one being a conunon connection and the other twelve being connections to the individual microphones. stutling box 54 is provided in the head of the eel to take the strain of the cable and to prevent leak. The forward end of the rubber tubing 51 which forms the eel body has an internal flange which fits in a groove. formed in a rearward exc fiu f the eel head. The rubber is securely held in this groove by a metal band at). The eel is provided with a tail ((30) secured to the end of the rubber body in the same way as the head. The upper iin 61 of the. tail is hollow and contains air while the lower fin is weighted. This keeps the eel from turnin over in the water and thus it prevents rolling carbon in the microphones. The hollow-space inside ol the rubber tube 51 is filled with water. and when so filled the eel has a neutral buoyancy so that it may be towed through the water without tendency to go to the bottom or come to the surface. It is found -that when the eel is towed at speeds from ten to fifteen knots the weight of the cable will keep the eel below the surface. At speeds above this it is advisable to add extra lead weights to the cable. In ordinary practice the eel is towed with about four hundred feet of cable out from the ship.

The microphone housings indicated generally by reference numerals are: spaced along inside of the eel body. Each micro-- phone housing 70 comprises a cup-like body of india rubber 71 generally of a cylindrical form provided with a plurality of radial flanges 72 in which are formed a circumferential groove 73 which does not, however. reach the bases of the flanges. The housing 70 is held in the eel body as shown in Fig. 4. An internal rib 7-l formed on the interior of the rubber body tube 51 fits into the circumferential groove 73 in the llangcs72. This holds the housing 70 in place, A metal band countersunk into the outside of the eel body, securely retains the rib 74 in position in the groove 73. The spaces between the rib 74 and housing 70 between the flanges 72 permit the conductor wires to be run by the several housings 70 also permit the water to run by the housings when the eel body is being filled.

l'n the eel shown in Fi 2 there are twelve microphones spaced twelve inches apart, the position of the microphones being indicated by the twelv metal bands 75.

'hen the eel is assembled the rubber tube 1 is placed in an outer iron pipe of an inside diameter about an inch greater than the outside diameter of the tube 51. 'The ends of the tube project a few inches from the ends of this iron pipe and are turned over the ends of the pipe and the air is exhausted between the siu'rounding iron pipe and rubber tube, thus expanding the tube. The housings 70 are held properly spaced by long, slender. iron rods lying between the Ilaugcs 72 and are then put in the expanded rubber tube. The air is then allowed to re-ente between the tube 51 and the sur rounding iron tube and the ribs 7-1 shrink into plac around the housings 70 and the iron spacing rods are removed. The tube 51 is then stretched lengthwise which reduces its thickness somewhat. and the metal bands 75 are slipped i place. Then the head and tail are applied.

In assembling the microphone housings T0. the cup shaped rubber body 71 is drawn over a brass shell 80. This brass shell has a cylindrical shape as shown in Fig. 8, being open for its full diameter at one end and having its edges turned in at the other end to form the short inwardly extending flange 85. This flange is conical in shape being slightly inclined to a plane at right angles to the axis of the shell 80, so that the edge of the flange extends slightly to the left as shown in Fig. 8. The thick end 81 of the rubber body 71 forms the sound transmitting diaphragm. A metal microphone support 82 is molded into the rubber and carries the microphone 83 which is of the inertia type. The microphone is shown in detail in Figs. 9 and 10. The carbon plate is rigidly connected with the stud 91 which screws into the metal support 82 so that the plate is vibrated by the rubber diaphragm 81. The metal microphone shell 92 is supported from the stud 91 by means of the double mica diaphraglns 93 and 94. The metal shell 92 carries the second contact plate 95 of the microphone cell. The carbon granules 96 lie between the plates 90 and 95. The metal shell 90 with the plate 95 forms the inertia element of the microphone. When the diaphragm 81 vibrates under the received sound the contact plate 90 vibrates with it. The rest of the microphone, however, tends to remain stationary because of its inertia, and the carbon granules 96 are subject to alternate pressure and release thus giving the resistance variations to set up undulating currents in the microphone circuit.

The several microphone housings are all of uniform quality and construction. The method (if assembly insures that the diaphragms 81 are all under the same tension and have the same time constants. scmbling the housing the brass shell 80 is heated and covered with a rubber cement. Then the rubber casing 71- is, drawn over the brass shell and the edge of the flange 85 pressed hard against the inside of the diaphragm 81. 1

In asi the shell 80.

The operator grasps the flanged part of the rubber and in pulling hard against the flange 85, stretches the rubber and reduces its thickness near the flanged end of the shell. \Vhile thus stretched a continuous metal band 86 is slipped over the rubber, which is then released. \V hen released the rubber contracts longitudinally and expands against the band which thus firmly grips the-rubber against the brass shell. The brass shell is preferably formed with slight beads spun in it to give a better grip in the rubber. The rubber between the band 80 and the flanged end of the brass shell is left under a certain amount of tension, which holds the inside of the diaphragm firmly seated against the flange 85. It is found that in applying the metal bands in this manner the diaphragms are put under a more uniform tension than is the case when a split metal band is applied and its ends drawn together and secured. lnthclattercase the rubber is drawn around with the band as it clamped and fits the diaphragm under a certain amount of distortion. The stretching of the rubber and the application of the continuous ring in the manner described not only gives a uniformly distributed tension over the individual diaphragm but secures a uniformity among the diaphragms of the several housings.

The flange 85 may be omitted, in which case the edge of the diaphragm is drawn against the end of the cylindrical body of The internal vibrating area of the diaphragm is that of the shell 80.

It is found that with the rubber dia phragm drawn against the end of the shell in the manner above described with or without the flange 85, the diaphragms of the several housings are uniformly seated against the brass shell and have a substantially equal tension, pitch and time constants.

The flange 85 is usually preferred in housings for submarine listening devices, because the flange 85 supports the diaphragm 81 and enables it to withstand the hydro static pressure of deeper subn'iergence without rupture. It also gives a higher natural frequency to the diaphragm and eliminates the lower pitched components of received sound which are characteristic of so-callcd water noise, causing the higher pitched sound components which are characteristic of the rythm of a ships propeller to be heard more prominently.

After the housings are assembled, the micropliones 83 are screwed on the supports 82. These microphones have previously been tested and are all selected to have the same time constants and approximately the same loudness. It is found that by using previously matched inertia microphones mounted as described. it is possible to get substantially the same time constants for each unit and that much greater uniformity can be thus obtained than has been obtained with the use of the pressure type of microphone in which some contact has to be provided or the stationary microphone plate.

The microphone is enclosed in an air space 100 in the housing which is sealed up water tight by means of a sandwich packing. This sandwich packing comprises two brass plates 101 and 102 with a layer 103 of soft india rubber between them. The plates 101 and 102 are pressed together by means of a screw 104 and nut 105 expanding the rubber 103 into a tight fit against the metal shell 80 and around the lead wires 106 and 107. An internal rib 108 is formed on the metal shell 80 and the brass plate 101 seats against this abutment. The water pressure against the packing tends to crowd the plate 102 inwardly still further compressing the rubber layer 103 so that the greater the pressure the better the seal.

One of the leads 107 is the common ground connection indicated diagrammatically at 14 in Fig. 1, while the other lead from the microphone forms one of the individual leads indicated in 1 2 etc., in Fig. 1.

The eel when towing behind the ship has a base line parallel to the ships keel and by a suitably calil'irated compensator the direction of the sound can be determined.

\Vhile the eel shown in the drawing has twelve microphones any number of microphones may be used. For example so-called two spot eels having two microphones one at the head and the other at the tail, have been built.

If a single eel is towed the readings on the compensator will indicate the angle of the sound to the base line of the eel, but with a single eel this angle may be either from the port or starboard side. The port or starboard ambiguity may be eliminated by towing two eels spaced several feet apart with units in both eels connected to a compensator, as will be readily understood by one skilled in this art.

The soft rubber walls of the eel allow the sound waves to pass freely from the surrounding water to the body of water within the eel and to the microphone housing diaphragms. The soft rubber eel body is free from resonance and does not change the character of the sound. Neither does it in troduce resonant sounds of its own when towed through the water, as is often the case with rigid towed devices.

The eel is flexible throughout its length and follows without yawing the pull of the towing cable. The flexibility prevents skidding or yawing to one side when subject to cross currents or swirls in the water. This makes it possible to tow two eels spaced apart five or six feet and have them run side by side with substantially constant spacing and not cross each other and tangle the cable lines.

The flexibility of the eel renders it more rugged to rough handling than rigid towed devices, as there is nothing to be permanently bent out of alignment, and the flexible eel body will always straighten out when towed and run true in the water, whereas if a rigid towed device is bent even slightly, it will yaw to one side when towed at the end of a cable.

The eel has a long stream line body and offers for its cubical capacity a minimum of resistance to towing. It is free from water noise when towed. It has not-hing to catch obstruction as it can be dragged through seaweed or along the bottom without catching.

The head, the tail, the microphone housings, and the body of the eel when filled with water each have a neutral buoyancy, which makes the eel ofa neutralbuoyancy,through out its length and gives it a uniform lineal density. This causes it to maintain a horizontal position in the water even when drawn slowly along, as for example from a ship drifting in a slight wind. The uniform and neutral linear density of the eel with respect to the sea water prevents jarring of the various parts with respect to one another due to unequal acceleration of the various parts.

While the microphone lines have been illustrated as arranged in a towed device such as an eel, it is obvious that a multiple microphone line may be otherwise constructed. For example a plurality of microphones 120 may be mounted as shown in Fig. 11 along the side of a ships hull 121.

The present invention is not limited to its illustrated embodiment but may be embodied in other structures within the scope of the following claims.

I claim:

1. A device for determining the direction of sounds comprising a plurality of spaced microphones and compensated connections for collecting the impulses from the microphones and bringing them to the ear 1n phase so that the impulses from the several microphones reinforce each other and produce a maximum.

2. A device to determine the direction of sounds comprising a row of spaced microphones and connections leading from a plurality of microphones at one end of the row to one ear and from a plurality of microphones at the opposite end of the row to the other ear, said connections having provision for the introduction of progressive time lag therein so that the impulse coming from several microphones may be brought into phase and produce a maximum and a binaural centering.

3. A device for determining the direction of sounds comprising at least four spaced mi(.-rophoncs, and compensated connections comprising paths variable in length leading from at least two of the microphones to each ear and serving to bring the impulses from the several microphones into phase at the cars so as to produce both a maximum and a binaural centering 4. A device for determining the direction of sounds comprising a row oi spaced microphones, connections from the microphones -l'or collecting the impulses from the several microphones and bringing them together and into phase at the car so as to produce a maximum, including means of introducing graduated time lag into said connections to compensate for an earlier arrival of the sound at their respective microphones.

A submarine sound detection device comprising an elongated flexible body arranged to be towed through the water and containing a plurality of microphones.

(3. A submarine sound detecting device comprising a long, hollow rubber tube arranged to be'towed at the end; ot' a cable and containing one or more sound detecting devices.

7. A submarine sound detecting device comprising a lon flexible rubber tube filled with water and containing one or more sound detecting devices. 8. A submarine sound detecting device comprising a long flexible rubber tube hav ing a series oi internal ribs formed thereon, and a series of microphone housings enclosed in the rubber tube and having grooves engaged by the ribs.

9. A submarine sound detecting device comprising two members one a long flexible rubber tube, the other a plurality of microphone housings thcrein, a circumferential rib on one of said members, and a corresponding circumferential groove on the other of said members to hold them in relative position, and a band on the outside of the tube to hold the rib and groove in engagement.

10. A submarine sound detecting device comprising an outer casing, and a microphone housing therein having radially extending flanges engaging the outer casing and leaving spaces between the outer casing and housing.

11. A microphone housingfor submarine sound detection, comprising a hollow rubber casing having a rubber body reinforced with metal on the sides and at one end, leaving one end free to form a sound transmit ting diaphragm.

12. A microphone housing for submarine sound detection comprising a cylindrical rubber body having a plurality of ribs formed thereon and adapted to act as the mountings for the microphone housing.

13. A microphone housingfor submarine sound detection comprising a hollow flexible body having an opening therein provided with an internal abutment, and a watertight closure for the opening comprising an inner plate seated against the abutment, a layer of flexible water-proof packing materia', and an outer plate,and means for t'orcing the plates together to compress and later ally cxtrude the packing material, the pressure of the water on the outer plate acting also to compress the packing material and make a tighter closure.

14. A microphone housing comprising a rigid shell having an open end and a cup shaped rubber body having a diaphragm at its end drawn over and against the end of the metal shell.

15. A microphone housing comprising a rigid internal shell having an open end with an inwardly extending flange restricting the area of the open end, and a cup-like rubber housing drawn over the shell and against the flange, the portion of the rubber housing which covers the opening forming a sound transmitting diaphragm.

16. A microphone housing comprising a shell having an open end and a rubber housing havin a diaphragm fitting over the open end of the shell and means for holding the diaphragm seated under tension of the rubber against the end of the shell.

17. A device of the kind described comprising-a shell having an open end, a cup-shaped rubber housing surrounding said shell in close contact therewith, the closed end of said rubber housing pressing against the end of said shell and formin r a diaphragm, and a rigid band surrounding the rubber housing and holding it against the outside of the shell whereby the diaphragm will be maintained in p'ace against the shell.

18. The method of assembling a cupshaped rubber housing over an internal rigid shell which consists in stretching the housing over the shell with its end pulled hard against the end of the shell thereby longitudinally elongating and reducing the thickness of the portion of the housing around the shell, slipping a band over the housing while stretched, and releasing the tension on the housing thus allowing it to expand and be gripped between the shell and the ring.

19. A submarine sound detecting device comprising a towed elongated flexible body having a substantially uniform linear density.

20. A submarine sound detecting device comprising a towed elongated flexible body having throughout its length a substantially neutral buoyancy with res met to the water.

HARVE T C. HAYES. 

